I think they are accounting for the less-speedy dogs by taking the average yps and then adding one yps to arrive at the sct, as Chris said in the video. I have a tall, moderate-speed dog ... on our cleanest, fastest, most efficient days we can hit just over 5YPS in classes like Tunnelers and Barrelers, for reference. Now, as an 11-year old veteran (back in her pre-veteran days we often didn't make time in classes like weavers and TnG) she is in the 12" skilled group - I took a look at the 12" times and they seemed quite doable to me. I'd have to go back and look at her pre-veteran YPS's to be sure (she's slowed down some but not tons), but the new times for the 16" dogs look ok too: for example, I looked at her scores from last weekend, and she would've made time as a 16" dog in all but two classes.
One problem with collecting all times is that without the oversight of an observer to weed out some runs, is that runs where dogs spent an inordinate amount of time sniffing, etc. would be included ... which would really skew the numbers in the other direction. It will be interesting to see how things are adjusted over time as more data is collected!